fi ment

Th roblems ^{0 un} table

r uree ^fi nag ment

PROGRAMME FOR THE DEVELOPMENT OF FISHERIES IN THE EASTERN CENTRAL ATLANTIC INT/81/014

FISHERY COMMITTEE FOR THE EASTERN CENTRAL ATLANTIC

CECAF/ECAF SERIES 84/28

THE PROBLEMS OF UNS TABLE RESOURCES

MANAGEMENT~/

by

S. Garcia

Senior Fishery Resources Officer Fisheries Department - FAO

1/ Lecture given at the DANIDA/FAO/ECAF Workshop on Fishery Management and pevelopment, Santa Cruz, Tenerife, 1-10/6/83

FOOD AND AGRICULTURE ORGANIZATION OF THE UNITED NATIONS UNITED NATIONS DEVELOPMENT PROGRAMME

Rome, 1984

of ItS authorltles, or concernlng the dei imitatIOn of ItS front/crs or boundaries

Ail rights reserved No part of this publication may be reproduced, stored in a retneval system, or transmltted ln any form or by any means, electronlc, mechanlcal, photocopying or otherwise, wlthout the prlor permission of the copyright owner, Applications for such permission, wlth a statement of the purpose and extent of the reproduction, should be addressed to the D,rector, Publications Division, Food and Agrtculture Organllatlon of the United Nations, Via delle Terme dl Caracalla, 00100 Rome, Italy.

© FAO 1.984

iii

NOTE FOR THE READER

The DANIDA/FAO/CECAF Workshop on Fisheries Management and Development was held in Santa Cruz de Tenerife from 1 to 10 June 1983. This Workshop (intended for West African fisheries managers-administrators, directors, economists and biologists) was to deal, among other matters, with the problems of instable resources, and the organizers asked me to deliver a lecture surnrnarizing the main conclusions of the FAü Expert Consultation

to examine changes in abundance and species composition of neritic fish resources (held 1n San José, Costa Rica, from 18 to 29 April 1983), centring the lecture as far as possible on West Africa.

The present document therefore contains a surnrnary of these conclusions as well as sorne personal elaborations on the consequences of this instability on modelization and management.

l hope to have paid justice to everyone through thebibliographic references, and would encourage the reader to refer to the Reports of the San José meeting for more

information~/.

S. Garcia

1/ Csirke, J 1983

and G.D. Sharp (eds.), Reports of the expert Consultation to examine changes in abundance and species composition of neritic resources.

FAü Fish.Rep., 291(1):102

v

TABLE OF CONTENTS

1. INTRODUCTION 2. BIOLOGICAL ASPECTS

3.

4.

5.

2.1 Variations in abundance 2.2 Other biological variations 2.3 Apparent replacement of species 2.4 Mechanisms of variations in abundance

2.5 Climatic variability in the eastern central Atlantic and its effects

NATURAL FLUCTUATIONS AND MODELIZATION 3.1 The production model

3.2 Stock-recruitment relation

3.3 The model of yield by recruitment 3.4 Prediction models

3.5 Bio-economic models

MECHANISMS OF BIO-ECONOMIC COLLAPSE 4.1 Biological collapse

4.2 Economie collapse MANAGEMENT PROBLEMS

5.1 Interaction between management and environment 5.2 Management strategies (following Csirke, 1984)

1 4 4

9 9 9 9 11 11 13 13 13 16 16 17 17 5.2.1

5.2.2 5.2.3

Controlled "fluctuating" fishing system Recourse to foreign fleets

Diversification

20 20 20 5.3 Management and reversibility of phases of collapse

5.4 Regulation methods

20 21 5.4.1

5.4.2 5.4.3

Direct regulation of fishing effort

Distribution of the controlled fishing effort Scientific "approximations" and management

21 21 25 REFERENCES

Annex Annex 2

26 29 30

LIST OF FIGURES

1. Variations in sardine landings in different regions of upwelling (Parrish, 1983)

2. Variations in catches of Alosa kessleri in the Black Sea (Ivanov, 1984) 3. Simultaneous variations in sardine landings (ref.)

4. Upwelling and catches of S. aurita in the Gulf of Guinea (data from CRO, Abidjan, FAO/CECAF, 1980)--

5. )

) Examples of replacements of species in the North Sea (Ursin, 1982), 6.) Namibia (Crawford, Shelton and Hutchings, 1983) and Peru (Jordan, 1983) 7. )

8. Variation of the biomass of herring and mackerel spawn and landings of sprat and lance in the North Sea (Jones, 1983)

9. Variations in sdrdine catches (._) and proportions of sardine (--) , horse mackerel and mackerel (- - -) horse mackerel, mackerel and sardinella (0---0). Source: CECAF Statistical Bulletin No. 3, 1981 10. Long-term climatic variations 1n West Africa (Faure and Gac, 1981) 11. Hydroclimatic variations in West Africa

12. Production model modifi,'c\ by wind section (and therefore by upwelling for the sardinella in Senegal) (Fréon, 1983)

13. Catches of sardines, and upwelling in Morocco (Belvèze and Erzini, 1983) 14. A: Standard production model

B: Production model affected by a climatic variable

15. Standard (A) stock recruitment relation and (B) affected by the environment

16. Standard (A) bio-economic model and (B) affected by the environment 17. Evolution of profits depending on costs (investments) and under the

action of the environment

18. A: Collapse and return of stock to normal

B: Biological eruption and return to normal with fishing and without fishing

19. Theoretical schematic representation of the evolution of biomass, additional eruptive production and fishing effort when a biological eruption occurs

20. Stocks of evolution of a fishery

21. Schematic representation of the Management/Environment interaction 22. Definition of annual quotas of anchovy in California (following MacCall,

1980)

23. Effect of interannual variability on the precision of the calculated quotas

..

2 2

~

3 5

6

6 7 7 8 8

10 12 12 14 14 14

15 18 19 22 24

1. INTRODUCTION

Since the end of the 1960s, the exploitation of fish stocks in the Eastern Central Atlantic from Morocco to Zaire has undergone considerable development. This development

is reflected in an increase in fishing effort, a diversification of gear, products and markets, and a considerable increase in catches - from about 900 000 t in 1964 to a maximum of 3 800 000 t in 1977. Scientific work in the region since intensive exploitation started has, despite the fairly limited means available, shown that there have been important changes affecting demersal and pelagie resources, their composition, their geographic

distribution and their abundance. The problem of the instability of stocks of small coastal pelagies was examined in depth at a technical consultation organized by _FAO~/, and certain ideas discussed at this meeting will be taken up again here.

Attention will be given successively to the biological aspects of the instability of stocks and their consequences on the traditional bio-economic modelization of exploitation, and on management methods.

2. BIOLOGICAL ASPECTS 2.1 Variations in abundance

It is important to understand that crises 1n fishery resources are not limited to particular stocks in the world, but occur in many regions, with a frequency that indicates a major problem to be taken into consideration (Fig. 1). Although catches are not a good indication of abundance, it is generally admitted that big fluctuations reflect mainly variations in abundance. The rapid rise in catches and their sharp fall after several years of intensive exploitation should be noted, particularly over the last two decades.

Figure 2 shows that these variations can take the forro of regular fluctuations over a long period (nearly two solar cycles, for example). Figure 3 shows that the fluctuations observed for stocks in various parts of the world can be synchronized, which would suggest that climatic factors play an important role in such fluctuations.

In the eastern Atlantic, very important variations have also been observed. The catches and the biomass of sardines have increased in the area to the north of Cap Blanc in an exceptional way (Belvèze and Erzini, 1983), whereas fishing grounds stretched about 850 km to the south (see Fig. 1 and Garcia, 1982). In the same region and especially to the north of Cap Bojador, unexploited stocks of oyster-catchers (Macroranphosus scolopax) underwent considerable fluctuations of biomass (ISPM,1983), providing, in the mid-1970s, a recently created potential of 400 000 t, which practically disappeared without exploi- tation at the beginning of the 1980s. Stocks of sardinella and mackerel simultaneously dropped sharply along the coasts of the Sahara, while the geographic distribution of

sardinella regressed toward the south, and a collapse of sardinella stocks and disappearance of mackerel were observed in the west of the Gulf of Guinea, from the Ivory Coast to Togo (Fig. 4). At the same time, triggerfish, a traditionally scarce species, developed bio- masses estimated at several hundreds of thousands of tons in the Gulf of Guinea and in the Sherbro-Cape Verde sector, even seasonally invading the northern coasts of Senegal as far as Mauritania (Caverivière, 1982).

Along Namibia, fluctuations seem linked to the collapse of sardine and anchovy stocks and the graduaI increase of certain stocks of horse mackerel (Fig. 1).

2.2 Other biological variations

In California, South Africa and Peru, it has been shown that variations in abundance are associated with variations in a large number of biological parameters such as growth, age of first sexual maturity, factors of condition, fertility, natural mortality, geogra- phic distribution, etc., indications of profound "mutations" in the population as a result of very heavy stress. These marked changes over the years seem to have been only li Expert consultation to examine changes in abundance and species composition of neritic

fish resources. San José, Costa Rica, 18-29 April 1983

2

Chile

Namibia (Wnlvis Bay)

1980 CEC\F (NOl<TH)

1970 1960

19)0 L940

1930

Ca1ifornia

, 11\

1 1

r

^{J 1}

,

^{1\\ "...}

, \ 1 \ l '"

1 \ , \ l "... \

1 " ....' \

1 \

1 \

'\ ,

^\

/ \ 1 \

1 \

" \... \ 1

,

~/

Fig. Variations in sardine landings in different regions of upwelling (Parrish, 1983)

solar activity

catches of Alosa kessleri

1925 1945 ₁₉₆₅

Fig. 2 Variations in catches of Alosa kessleri in the Black Sea (Ivanov, 1984)

3

Chi le

",

Jnpan

_, , , ---

^C.:J1ifornia

_, ,

₁¹

^, , , _,

1

,

1

,

, ,

, ^,

, _,,

, ,

, _,

en

, ,

eu

,

...c:: 1

,

0

,

+-l

,

<Il 1

,

u

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, ,

,

1900 1920 1940 1960 1980

Fig. 3 Simultaneous variations in sardine landings (ref.)

catches

upwe11i ng (arbitrary scale)

lOO

enç:

75

0 +-l C""l

0

:<

50

' - '

eneu ...c::

0 +-l

<Il

25 u

Fig. 4 Upwelling and catches of S. aurita in the Gulf of Guinea (data from CRO, Abidjan, FAO/CECAF, 1980)

anothar aspect of variability, whose importance at geographica1 1eve1 has a1ready been pointeJ out, since variations in biologica1 parameters are a1so considerable from one region to another. A1so, it is now wel1 estab1ished that the gregarious behaviour of these species 1eads to significant changes in their catchabi1ity (which increases when the biomass drops) •

2.3 Apparent replacement of species

It has been suggested rnany times that the biomass produced by an ecosystem is more constant than that of the specifie e1ements that compose it, or in other words, the species whose stock has collapsed will be rep1aced by another. The case of the sardine and the anchovy of Ca1ifornia has become a c1assic case used to expound the theory of interspecific competition. However, more recently, MacCall (1983) contested the hypothesis of "replacement"

of species in the strict sense (implying phenomena of interspecific competition) in the case of California, and considered it more likely to be a question of important changes in the biomass and the geographic distribution of resources, as a result of environmental action.

There are many examples of apparent replacements (Figs. 5 to 9) in most regions of the world, and the eastern central Atlantic has also been the scene of very significant replacements (Garcia, 1982).

Between Cap Blanc and Cap Barbas, the replacements observed with small pelagie species are due to changes in geographic distribution and in stocks (Belvèze, 1983). It is nonetheless true that these movernents, in the case of sardine and triggerfish, are accom- panied by considerable increases in the biomass of these species, to the detriment, it seems, of other species (Caverivière, 1982; Gulland and Garcia,1984). It is important to note that these replacements, which do not always appear as irnrnediate or autornatic, can be advantageous, like the replacement of the Sparidae by the Cephalopodae in the northern CECAF zone, or disadvantageous, like the replacement of a traditionally marketed species by triggerfish throughout the Gulf of Guinea.

2.4 Mechanisms of variat ions in abundance

Important work has been done in describing variations in resources and atternpting to understand their mechanisms, in California (McCall, 1983), in Peru (Jordan, 1983), in Japan (Hayasi, Tanaka, 1983), etc., and in Morocco (Belvèze and Erzini, 1983), Senegal (Fréon, 1983), the Ivory Coast and Ghana (FRU/ORSTOM, 1975; FAO/CECAF, 1980; Binet, 1982; Gulland and Garcia, 1983). The Expert Consultation organized by FAO in April 1983 discussed the subj ect at length (Sharp and Csirke, 1983).

Although there are still divergences of op~n~on as regards the detailed mechanisms, it is now widely admitted that these variations are linked essentially to climatic fluc- tuations whose consequences are aggravated by excessively intensive exploitation.

Clirnatic variations are probably reflected in variations in the rate of survival of recruits, and therefore, their abundance. Variations, which are reflected in short series of good or bad recruitments, are accompanied by variations in sexual maturity, growth, fertility, natural mortality, etc. The stock-recruitment ratio is also affected, and it may be said that variati.ons in abundance are linked to changes in the biological capacity of the environment vis-à-vis the larva stage.

It is not impossible that interspecific competition, predator-prey relationships, or the side effects of fishing (on another species than the one whose fluctuations are under consideration) may also have an influence, but this has never been satisfactorily demonstrated, whereas the effects of the environment are often obvious.

Information collected from sediments on the abundance of stocks of sardines and anchovy in the past, have shown that biological collapses took place, practically without any exploitation, over the last two centuries (Soutar and Isaacs, 1974).

1

5

1980 herring/mackerel

"""'--

1970 1975 1960 1965

Fig. 5 - North Sea Fig. 6 - Namibia

~

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ue

oJ:) 12

~

~

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QJ

..c:u u nlu

A n c h o v e t a / ' \

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. / .

Bonite

Figs. 5, 6, 7 Examples of replacements of species in the North Sea (Ursin, 1982), Namibia (Crawford, Shel ton and Hutchings, 1983) and Peru (Jordan, 1983)

1965 1970 1975

Fig. 7 - Peru

..

rnackerel

100

80

20

o

l<'05~€.s

1962 1966 1970 1974 1978

Fig. 8 Variation of the biomass of herring and mackerel spawn and landings of sprat and lance in the North Sea (Jones, 1983)

t

Fig. 9 Variations in sardine catches (---) and proportions of sardine (---) , horse mackerel and mac~: , ( - - -) horse mackerel, macke,-, ld sardinella

(0---0). Source: CECAF ~tatistical

Bulletin No. 3, 1981

4-<

o

albOl-<

1-< al Cll ;>

.c:^.,4 Uo::

'Il .,4 ...

"Cl Cll ... bOal

<Il J:

~ al J:(fJ

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7

Arid

1880 1900 1920 19:'0 1960 1980

A

fil

"Cl .,4J:

~ al

"Cl

<Il 1-<

'"'

4-<

0

"Cl al alp..

(fJ

Fig. la Long-term climatic variations 1n West Africa (Faure and Gac, 1981)

Speed of winds in Senegal Anomaly of salinity in the

Ivory Coast

B

fil

"Cl

1

^CllJ:

r:~~~~J~~::=j~~~\

^{n -_ _}

1-<

'"'

4-<

o

"Cl al alp..

(fJ

1947- 194R

1955- 1956

1965- 1966

1975- 1976 Fig. 11 Hydroclimatic variations in West Africa:

A - Anomaly of salinity at Abidjan (FAO!CECAF, 1981) and speed of trade winds in Senegal (Fréon, 1983) B - Index of upwelling in Morocco (zone B), (Belvèze, 1983)

and speed of winds in Senegal (Fréon, 1983)

c ^{:>.6 mis}

4.6 mls mis

mis 6

4 J)

B

A

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;::

U '-'<Il U

Il D C

10 'i 10 4

Fishing effort (10 hours) Fig. 12 Production model modified by wind section

(and therefore by upwelling for the sardinella in Senegal) (Fréon, 1983)

'"

;::C1J '-'U

<Il U

e Fig. 13 Catches of sardines, and upwelling in Morocco (Belvëze and Erzini, 1983)

_e

Upwelling index

9

It is however, admitted that fluctuations are aggravated by fishing, which also considerably increases the risk of collapse.

2.5 Climatic variability in the eastern central Atlantic and its effects

Analysis of this variability is interesting, since it indicates the type of varia- bility to be expected at the level of the resource. Figure la clearly demonstrates that one of the phenomena marking the climate of West Africa is the existence of a very clear cycle of Sahelian droughts. These droughts are linked to movements of atmospheric fronts, and cause considerable variations in river discharges, and therefore in the salinity and primary productivity of the coastal areas (Binet, 1982).

They are accompanied by major changes in the speed and direction of prevailing winds, particularly winds in upwelling regions (Figs. 4, llA and llB), which are also reflected in major variations in primary productivity. Figures llA and 11 suggest, moreover, that hydroclimatic variations observed along the coast and over the Atlantic are perhaps corre-

lated. The variations appear similar or opposite depending on the case, and taking into account the shortness of the time series. This hypothesis must, however, be considered with extreme caution. Figure llB indicates that apart from the medium frequency fluctua- tions, there are also long-term trends toward a weakening of the upwelling (Senegal) or its strengthening (Morocco).

The work of Belvèze and Erzini (1983) in Morocco, and Fréon (1983) in Senegal, indicates that variations in abundance of stocks are linked to the environrnent (Figs. 12 and 13). Fréon has even proposed a production model for the sardinella of Senegal, including environmental fluctuations.

3. NATURAL FLUCTUATIONS AND MODELIZATION 3.1 The production model

This model schematizes relations between catches, cpue and fishing effort. Tt is weIl known and widely used throughout the world, in particular in fishery commissions.

Although it is criticized at present (Larkin,1977. Sissenwine,1978), it will remain for at least several years more an essential tool, for lack of a better one. Tt is there- fore useful to study the consequences of instability of resources on accepted theory.

We shall not spend too much time on the effect of variations of catchability (linked to variations of biomass) on production models. These variations, which tend to produce a distorted model, with an overestimation of the effort corresponding to maximum production and an underestimation of the increase in mortality due to fishing

thus leading to severe overexploitation, have been studied elsewhere (Fox, 1971).

When the environment has a marked effect on production, the production model describing the reaction of the stock to exploitation can no longer be envisaged as a simple determinist function (Fig. 14A),but as a family of curves corresponding to ambient conditions and the different biological capacities of the environment (Fig. 14), or as a multivariable model:

Catch

=

f (effort, environment)

The course of the fishery through this family of curves becomes complex and not automatically reversible. There is no longer a single maximum sustainable yield (MSY) but several, depending on ambient conditions; and the classic notion of MSY has no longer any sense except in the very long term. Tt has been demonstrated that, when the variations are significant, the maximum average yield (MAY) really obtained will be below the MSY

(Sissenwine, 1978).

3.2 Stock-recruitment relation

This model, widely used (but seldom demonstrated) in standard works, depicts the quantitative relations possibly existing between the size of a parent spawning stock and

QI ::::>

0-u

Effort

CIl

..cQI () +J Cl!

()

Effort

><

S2

....

QJ

=>

uD-

Effort

o

Fig. 14 A: Standard production model

B: Production model affected by a climatic variable

The points represent the theoretical course of a fishery.

Figure 14 B illustrates the fact tha' in this case the course is no longer necessarily reversible.

11

the importance of the resulting offspring (or recruitment). An examp1e of the standard relation (Ricker type) is given in Figure 15A. The impact of natural variability on this model has been discussed at length by Garcia (1983). lnstead of such a determinist mode1,

it is necessary to consider a multivariable model or a family of curves (Fig. 15B), each one corresponding to an ensemble of given ambient conditions. In these conditions, there is no longer one single point of equilibrium (El, Fig. 15A) for the fishery at the inter- section between the curve S~,R (intrinsic relation between the stock and the recruits) and the curve R~S (trivial relation between the recruits and the resulting stock, of which the slope is a function of F, morta1ity by fishing) but a whole series of virtual equilibrium points (El to ES on Fig. 15B) situated along the line DA. In other words, the stock will vary from one year to another in an autocorrelated manner, because of

changes in recruitment, even if the effort remains unchanged and the notion of equi1ibrium becomes pure1y statistica1.

In the same way, it may no longer be considered that there is characteristic level of mortality by fishing F (criticial) for which the function R-+S occupies the u1timate

position at which the rate of replacement of the stock becomes insufficient and the stock collapses (Fig. 15A). lt should, on the other hand, be admitted that the risk of co11apse is always there, even for virgin stock, as is shown by observations on the California sardine in the past (Soutar and lsaacs, 1974) and the simulations of Laurec, Fonteneau and Champagnat (1980). A high risk zone could also be defined (hatched section in Fig. 15B) in which the fisheries administration could estimate that the bio-economic risk became insupportable.

lt should be noted that, when catchability q varies according to the biomass (parti- cularly if it increases when the biomass decreases), the replacement line R~S (Fig. 15B) will swing toward the left because of an increase in mortality by fishing F (not linked

to an increase in the fishing effort) and will tend to approach the critical F zone, where risks of collapse are very high. These natura1 variations of the biomass, accompanied by variations of q, therefore involve important variations of the risk of collapse even if

the fishing effort does not increase.

Moreover, the natural variations of q and therefore of F will also be ref1ected in variations of total catches, even in the absence of variations of the nominal effort because of the relation between F and catches recognized in the production mode1. In this case, there is confusion between the effects of the environment and those of fishing.

When the natural variations of recruitment are considerably superior to those produced by fishing and by changes in the size of the spawning stock, the stock-recruitment ratio could be very simply replaced by a model R = f (environment).

3.3 The model of yield by reeruitment

This model is usually considered as being unlikely to be affected by climatic varia- tions, sinee it is not affected by recruitment variations. lt has, however, been demons- trated that natural variations of abundance are accompanied by variations in growth, natural mortality, maturity, etc. lt should therefore be admitted that this model will also be affected, which could lead to a revis ion of the rules as regards age nt first catch, for example.

3.4 Prediction models

The recognition of the instability of an important natural resource immediately raises the problem of its continued monitoring, for examp1e, by acoustic methods. lt a1so raises the problem of prediction, since it then becomes fundamental ta foresee in advance the evolution of the abundance so that the necessary management measures may be taken in time (prediction of R, of the biomass, of catches). Prediction models can be researched using relevant characteristics of the environment as predictive variables (cf. Garcia and

LeReste, 1981; LeReste, 1982). They are, however, only useful if they really make possible a prediction sufficient1y far in advance to enable industry and fisheries administration ta react, and if the precision of the predictions is sufficient.

o'"'---~

Size of stock

Increase of

B

A

o---:---.:::::==~~Size of stock

F1g. 15 Standard (A) stock recruitment relation and (B) affected by the environment

A

...,

fil fil 0 U

"0 ç:

(1j fil Il!

...;:l

:>(1j

B

Effort

Fig. 16 Standard (A) bio-economic model and (B) affected by the envi ronment

13

3.5 Bio-economic models

The consequences described in Paragraph 3.1 concerning the production model can be expanded to the global economic model, widely used in fisheries economics. The standard model (Fig. 16A) assumes that there is an equilibrium point Z at which the development of

the fishery will stop, and where the economic profit will be nil. The existence of natural variations in abundance involves the theoretical existence of a family of bio-economic models (Fig. 16B). There is therefore no longer a point but a line of equilibrium OA along which the fishery will tend to move when the stock passes from the abundance corres- ponding to the lower curve to that corresponding to the higher curve. The essential con- sequence is the existence of major fluctuations in profits (Fig. 17), and especially the possibility of exceptional increases, even in the case of overexploited fisheries. The natural fluctuations in abundance (and in particular the increases, regular or not, in the biomass) will therefore be powerful incentives for the exercise by professionals of very strong pressure on governments to obtain the aid necessary for expansion and recon- version (from the fresh fish market to meal, for example). They will finally be the source of overinvestment reaching astronomical levels, and out of aIl proportion to the levels of overinvestment that a more stable resource is able to generate in the context of a stable market.

4. MECHANISMS OF BIO-ECONOMIC COLLAPSE

A distinction may be made between biological collapse and the economlC collapse resulting from it.

4.1 Biological collapse

There has been for a long time a tendency to consider that a rapid and large-scale decrease in the biomass was abnormal and represented a collapse (Fig. 18A). Measures to be taken therefore aimed at reconstituting the stock. The examples given in Paragraph 2.1

show that the biological reality is rather the existence of "eruptions" of the biomass (regular or not) followed by a return to normal in the absence of intensive exploitation or an earlier return to a level below normal in the opposite case (Fig. 18B).

Figure 19 analyses the evolution of such a biological eruption, represented schema- tically by an evolution of the biomass (B) following a normal curve. Figure 19A shows the theoretical evolution of B with or without exploitation over a cycle of several years in decades. Figure 19B shows that "eruptive biological production" is positive in the growth phase and then negative, passing through a maximum during the growth phase.

Figure 19C gives a theoretical representation of the development of fishing effort (and land capacities) parallel to the biological eruption, in the absence of regulation.

Experience shows that this effort continues to grow after the biomass has passed the maximum, reaching its peak when the eruptive biological production is already strongly negative.

This simplified approach implies:

(a) that no management is in a position to force the stock to any given equilibrium at the high levels of biomass observed during eruption, or to stabilize the fishery when the collapse has started.

(b) that intensive fishing, above the maximum of eruptive biological production, adds a factor of decrease of the spawning stock at a particularly critical moment in its evolution. It leads to far lower levels of biomass than those to which the stock would have descended for exclusively natural reasons. It therefore endangers the dynamic equilibria established by the species during its evolution.

It is important in this connection to note the difference between the consequences of natural variations in abundance of stock and those due to fishing. In particular, the age structures will be different. A stock reduced for natural reasons (drop in recruitment) will be composed of individuals that are old and therefore of comparatively high fertility,

8

Fig. 17 Evolution of profits depending on costs (investments) and under the action of the environment. The 4 curves correspond to the 4 parabolas of Fig. 16B. The arrows indicate the course of an uncontrolled fishery when the biomass (and the profits) increase and then decrease. The equilibriul'l points 21 to 24 (profits = 0) correspond to those of Fig. 16B

CIl Uc::

ni

"

^c::

15<:

Qi

c::U ni

"

^c::

.D<:

8

normal level (without fishing)

normal level (with fishing)

Time Fig. 18 A: Collapse and return of stock to normal

B: Biological eruption and return to normal with fishing (lower curve) and without fishing (higher curve)

15

Aver,lgl'

hi(1I1l.lsS

lncrease

Time

'l'1ml"

average F! Z l nll;:',-lerm with flshing 'vitllOut fis!ling

OVl'rcap:lC'llv

C

^F/z

A

rJJ rJJm

B

E0

®

.,.<

A 4-<

0 rJJç::

0

0

.,.<

~m

.,.<

e

1-<

:>m

biological maximum

economlc

maXlmum Time

Fig. 19 Theoretical schematic representation of the evolution of biomass.

additional eruptive production and fishing effort when a biological eruption occurs

whereas an overfished stock (drop in life expectancy) will be composed of young and not very fertile individuals. It should therefore be admitted that the reconstitution capacities of a stock, constituted in the course of evolution by adaptation to the environment, will be adversely modified by fishing.

4.2 Economie collapse

The appearance of a biological eruption is a major signal in the ecosystem exploited, which industry detects rapidly and to which it replies immediately if the market is ready for it. It has been seen in Paragraph 3.5 that the profit increases rapidly. The search for larger, short-term profits leads the fishery system to hypertrophy. Fishery adminis- trations submitted to strong pressure often help rapid development by establishing favou- rable investment codes and national or international development banks. The resulting decrease of the biomass will be partly compensated for by improved techniques (sonar, sounder, "pack" fishing, use of aeroplanes, etc.) increasing still further the catch over- capacity, and leading to levels of mortality due to fishing several times higher than the natural mortality to which the stock is adapted. Markets will also be changed. For example, a big market for fish meal could be developed, making it possible to process the exceptional quantities caught, and leading to a change in processing factories on land.

Figure 17 clearly shows the non-reversibility of the process when the eruption is over and the biomass decreases sharply. The protits become negative and bankruptcies quickly follow. The reduction of fixed costs is slower than that of current costs. The gregarious behaviour of pelagie stocks involves high mortality even when the fleet is reduced.

Management measures discussed in a climate of crisis are taken too late and not properly enforced. Favourable fluctuations in markets as a result of the reduction in supply can perpetuate for sorne time the adverse effects of the system, until demand is directed toward a replacement product (soya, for example).

In the end there is a total collapse of a hypertrophied economic actlvlty, with disastrous financial consequences (boats withdrawn from fishing, factories closed, staff out of jobs, serious economic crisis). The drop in supply over a long period can involve a loss of markets which is difficult to reverse.

Economie collapse raises serious problems of reconversion of the whole sector and implies enormous State expenditure (subsidies, nationalizations, various set-offs, high social costs).

The consequences of economic collapse are felt outside the country in the markets, but even more so through large-scale transfers of ships and factories available at low priees. The development policies of countries with resources still underexploited can in turn be seriously affected by this international situation, in an insidious way (cf. the chain of collapses in California, Peru, South Africa/Namibia).

5. MANAGEMENT PROBLEMS

Traditional management methods are mainly based on the theoretical notion of long- term stability of resources. In these circumstances, the exploitation system must find an equilibrium determined by fishing costs and value of catches. Management problems really come down, so to speak, to defining a level of effort where it is possible to reach a

number of objectives, and to stay at that level despite a continuous increase in fishing efficiency.

In the case of variable, or unstable stocks, the notion of equilibrium only has any meaning, at best, in the very long term, if it is admitted that a maximum average yield exists, but it is practically useless for management. Rather than the time scales which are relevant for development (5 to la years), it should be considered that certain pelagie stocks are in permanent disequilibrium. This involves the risk of uncontrolled development of the fishery, with disastrous consequences for the economy if the stock

17

collapses (cf. anchoveta stocks of Peru, sardine of California, pilchard of Namibia, etc.).

The example of the collapse of Ghana's sardinella stocks seems to indicate that the consequences are less serious in the case of flexible small-scale fisheries with multi- specifie resources.

The main problem for the authorities responsible for managing a fishery with risks of instability is in fact that of regulation of investment, at sea and on land, if stocks become increasingly abundant over a number of years.

The alternative lies between, on the one hand, uncontrolled hyperintensive exploita- tion of the biological eruption, leading to irrnnediate high profits followed by very high costs for the State and the taxpayer; and, on the other hand, a wise and controlled exploitation policy, extracting only part of the exceptional resources produced by the eruption, but in compensation reducing to a maximum the negative effects and the costs engendered by overcapacity. Bearing in mind the unfortunate consequences of an economic collapse, we will study only the possibilities of controlling the development phase of the exploitation in the case of biological eruption.

5.1 Interaction between management and environment

The traditional descriptive diagram of the graduai development of a fishery is shown in Figure 20. When the effort is the principal factor of change in the size of the stock, the fishery will pass through stages 1 , 2 and 3 to develop and possibly through stages 4 and 5 if the fishing effort is not effectively controlled.

When the environment plays an important role in the determination of recruitment, and therefore of the size of the stock, the development of the fishery will be conditioned by the interaction between the environment and management, and the risks of collapse will be aIl the greater if the variations of biomass linked to the environment are important and the fishing effort develops rapidly. In the event of unfavourable circumstances

arising, the evolution in 5 phases will be stopped and the expansion phase (Stages 1 and 2) will be followed by a stage of collapse.

The following organizational chart (Fig. 21) gives a very schematic representation of the process of interaction between management and environrnent during the ~ourse of a fishery, from Phase 1 (underdeveloped)toPhase 5 (collapse) and possible reconstitution.

This is a simplified diagram, of which a more realistic and more complex version is given in Annexes 1 and 2. It is, however, enough to show that in the absence of regu- lation of effort (direct or indirect), there is a high risk of unstable stocks collapsing.

It also indicates that risks are reduced but not suppressed by management, and that they exist even in the absence of fishing, although to a lesser extent.

ln the diagrams shown in Annexes 1 and 2, consideration is given to factors such as profitability, the action of the environment on the survival of larvae, the need to conserve a minimal reproductive biomass, the possibility of blocking the fishing effort, but also that of optimizing it permanently, compensating for the inevitable gains in efficiency, or by regulating mesh sizes and fishing seasons, for example. The variation of catchability depending on the biomass, an important factor for small pelagie species, is also considered.

These diagrams are obviously, despite their apparent complexity, simplifications of reality. They do not take into account aIl the human factors and the factors of multi- specifie interaction. They do, however, offer a representation which illustrates sorne of the mechanisms leading to the instability of certain fisheries, and the high risk created by the combination of variable biological production and the absence of effective control of catching methods.

5.2 Management strategies (following Csirke, 1984)

Let us take the viewpoint of a coastal State managing a resource that belongs to it.

The fishing system, like a predator confronted with the problem posed by a variable resource in abundance, could in theory react as follows:

~

¹

~ ^'"

₁

0 1

i ^Il

o .... 0

j ;,

®

~

¹

~

²

z '"

le

¹

~

~ 0

II ~

CD' ~A

¹

~~! ~

II~· ^!

¹

^{, !}

^Time

<l>

c:U III 'tlc:

;:l .D<

./1 ^!~

[

1

'~

I~

F i i ^{1 / ' \ ; Time 00}

Time

Normal evolution

Hyperdevelopment

(interaction betw('(~nmortality by fishing dnd environml~nl)

Fig. 20 Stocks of evolution of a fishery.

Thick lines: traditional situation where the effort acts alone on the size of the stock

Thin lines: hyperdevelopment situation linked to an eruptive phenomenon ThE horizontal l ines A. B, C and D represent pseudo-stationary states of the fishery

19

DEVELOPED

Low risk

OVERFISHED

UNDEVELOPED

COLLAPSED

FaVUlIrablL' environmen t'?

F~lV()llrilh1L'

eLU110my?

Ef[L'CtiVl'

m.ll1:lgement?

OYes

( A )

(E)

(ECON)

Fig. 21 Schematic representation of the Management/Environment interaction.

The 6 phases numbered from 1 to 6 refer to Fig. 20. The pseudo- stationary states are represented by hatched areas - (Source:

Csirke, 1984) - See also Annexes 1 and 2

vary its own numbers (and fishing power), with a certain time lag;

stabilize its numbers by finding substitute prey, whether of the same type but elsewhere (migration), or of a different type but in the same place (change of prey).

5.2.1 Controlled "fluctuating" fishing system

The national system of exploitation would be authorized to vary its fishing power, but in the framework of well-defined constraints, to avoid a damaging overcapacity.

Figure 19 shows (at the bottom) that fishing effort could be increased at the beginning of the ascending phase when the income grows, but that it should begin to decrease before the peak of the biological eruption is reached.

It would be economically preferable to use the technological flexibility of exploita- tion to increase the catching ability of the fleet without increasing the numbers of fisher- men. The regulation of effort could be obtained through a system of annual quotas (see Paragraph 5.3) in the context of a general limitation of fishing effort.

The level of fish processing raises particular problems because it is more rigid.

An improvement in techniques is perhaps inevitable in the case of eruption, but large-scale changes, particularly irreversible ones, should be avoided.

At aIl levels, decisions should be based on awareness that a biological eruption is ephemeral, and the need to manage the fishery for the long-term good of the whole country.

The main difficulty for the authorities responsible for management is to resist pressure by industry, motivated by short-term considerations. A maximal ceiling for effort, not to be exceeded whatever the evolution of the stock, should be defined in advance.

5.2.2 Recourse to foreign fleets

It could appear preferable, to catch the exceptional production and the potential profits it implies, to use a fleet with a large radius of action (a "migratory predator").

The perception of fishing rights would make it possible to take advantage of the exceptional profit (the fish is not given away but sold on the spot) without investment. The conse- quence would, however, be a reduction in the additional direct profits that the national fleet could obtain if it exploited the same resource, because the yields obtained would be reduced by the foreign fleet. It may be said that in this case the exceptional profit is totally or partially appropriated by the State instead of by the national shipowners or, which is more serious, squandered in overinvestment.

5.2.3 Diversification

Just as a predator changes prey when its abund?nce falls below a certain level, so the system of exploitation could be developed, retaining sufficient flexibility to exploit without catastrophic changes in numbers, a collection of species whose total biomass is considered more stable than that of its specific components. This would in fact mean the optimization of a strategy that the fishing system already uses in the absence of management, but in a way which is sometimes inefficient and costly for the taxpayer.

It would be necessary in this case to set a ceiling for fishing effort globally, at a level considered sufficient to exploit aIl the small pelagic species availble (cf. Para- graph 5.4.1). It might be necessary to establish quotas per species, to avoid the concen- tration of the fleet on one species alone in periods of biological eruption.

5.3 Management and reversibility of phases of collapse

The history of unstable fisheries shows that usually, when the biomass decreases very rapidly, it is already too late to act and avoid the biological and economic collapse, if the phenomenon has not been anticipated (Fig. 19):

on the economic level, the problem is to save what can still be saved and to look for new terrains (in the widest sense) for exploitation;

~.

21

on the biological level, pressure on survivors must be reduced as far as possible; mortality of juveniles per fishery must be reduced (if possible to zero); sanctuaries should perhaps be provided (by closing fisheries altogether in coastal areas); any catch, even secondary

(moratory), of the species in question, should be controlled and, if possible, prevented. These measures would provide the best chances of reconstituting the stock. It is clear that, on the economic level, the revival of the system of exploitation after collapse will not be

achieved purely and simply by reversing the course followed during devel opmen t.

It is also clear that this return to normal implies, paradoxically, additional costs, usually for the State. On the biological level, it must be emphasized that the reconstitu-

tion of the stock does not seem to be either automatic or immediate. It seems that the reconstitution is easier if the collapse is minor (or if the biomass has not been forced belowa certain critical level).

There is a risk that, for a long time, the collapsed stock will remain at a very low level, more or less stable, even with Draconian management (probably difficult to implement efficiently), if the stock has been forced to a level of abundance where it is dominated by other species, or to a level where its spawning behaviour is disturbed, or again if

its reconstitution depends on the reappearance of specifie climatic conditions.

5.4 Regulation methods

The main management problem posed by unstable stocks differs from that of stable stocks, first of aIl because of the special difficulty of controlling the intensive development phase and the resulting dangers. Regulation of effort (and thercfore of development) could be envisaged in two different and complementary ways.

5.4.1 Direct regulation of fishing effort

The State should effectively control development of the fleet. This is not an attack on freedom of enterprise but an essential safeguard for the interests of the nation as a whole (which is left with the bill for the economic collapse). A maximum global level

("ceiling") of fishing effort should therefore be defined for all the small pelagies, and therefore a maximum fleet size, according to criteria to be defined. This ceiling could be revised periodically. A fixed periodicity, for example, every five years, would prevent excessive pressure being exercised on the administration for "continuous" revisions.

A quota system would be established.

5.4.2 Distribution of the controlled fishing effort

To avoid sterile competition and an arms race, quotas could be established by region and by port (by country, in the case of shared stocks).

To avoid concentration on one species, annual quotas per species could be established.

To adjust catch rates, annual catch quotas could be defined, using for example, the model proposed by MacCall(1980) for the California anchovy (Fig. 22). This model closely follows the standard production model, but is based on a different representation of the same theory, to the extent that biological production (and therefore the balanced catch) is represented directly as a function of the biomass and not as a function of fishing effort (Fig. 22A). Schaefer's linear model forecasts a parabola passing through a maximum for B

=

B

=

0.5 Boo , where Boo is the biomass of the virgin stock. Each point of the curve is ~~Xequilibriumpoint, and the line joining the origin at this point (OA) to a slope equal to C/B, that is to say, equal to F, mortality by fishing. It may be seen in particular in Figure 22A that the line joining the origin to the summit of the parabola has a slope equal to F

MSY (in reality, if catchability varies with the biomass, there would be a curve (OA')).

B,

U o ' - - I - - --'- B

.~"IH

c

8

" ' - - - ' ' - - - - ' - - - - ' - - - ' -

a.."..

.G;\

....

_.--

--~

CATCH

A

--I!:....---.JBl,_-'B~2--lB-_-.-=~B:-OO---'aoo=-::----~B

2

°2_-+Of---+-;--,~

0,_-l--~I--__,l1'_

E c

F

OL----lB~""-n--l-B-"""'---.L---"...- . . B

Flg. 22 Definition of annual quotas of anchovy in California (following MacCall, 1980) _ A: objective B: objective = 2/3 MSY - C: constraint of minimal biomass - D: various possible quota lines _ E: vectors of F resulting from D

MSY -

23

The advantage of this model is that it is directly expressed in terms of abundance.

If the objective of management is to fish to the level of the maximum theoretical production, the line DA could be used to determine the variable annual quotas Q , Q2' etc., in the

light of the changes of abundance B 1, B

2, as indicated in Figure

22A~/.

If the objective is to take out only two thirds of tne maximum possible to minimize biological risks, a line of quotas with a slope equal to two thirds of F

MSY will be chosen (Figure 22B) for more prudent management.

In the two cases described above, the authorized quotas change proportionately to the variations of the obverted abundance. A more conservative process of determination of quotas could be defined with, for example, a more rapid reduction of quotas than the decrease in stock and a nil quota as soon as the stock is below a predetermined value

(Bmin) defined, for example, in such a way as to conserve a minimum spawning stock (Fig. 22C).

ln this case the line of the quotas used (DA

l) would be parallel to DA and would lead to even more conservative quotas. In the case represented in Figure 22C, the quota Q corres- ponding to biomass B

l is nil since B

l ^<. Bmin. 1

This model can be used to define quotas for very complex strategies for regulating fishing effort. Figure 22E shows regimes of exploitation (F authorized depending on the evolution of the biomass) corresponding to different lines of given quotas in Figure 22D.

This allows for the possible need to reduce F by the strong values of B so as to conserve the very necessary large buffer biomasses, to conserve a minimal spawning biomass (which is one constraint of the model), to reduce F more rapidly than the biomass when the latter decreases, and to stop fishing entirely when the minimal biomass is reached, etc. This version of the model is in theory relatively robust if there is an error of determination of the equilibrium curve. In fact, the successive quotas will involve the displacement of annual couples (biomass, catch) approximately the whole length of the line of quotas, in search of the equilibrium point at the intersection of the latter and the curve.

If the theoretical equilibrium curve calculated is grossly mistaken, the points observed would be displaced toward an unexpected equilibrium point at the intersection of the chosen line of quotas and the equilibrium of true production, thus indicating the error made. The risk of seriously degrading the stock because of this kind of error is therefore limited. It is still more so if a constraint of minimal biomass is adopted.

This robustness is, however, only theoretical if it is admitted that the stock under- goes variations of abundance of natural origin. In fact, the model recommended by MacCall is determinist to the extent that it is based on only one production curve. In the case of important and autocorrelated fluctuations in the biological capacity of the environment, it may be estimated that there is no longer one curve alone, but a family of curves to represent the stock in its different states. With the procedure described above, annual quotas would therefore tend to be systematically over- or underestimated for several years, and a long period of observat~onwould be necessary before the form of the average long-

term curve could be defined. ln the meantime, these essential errors could be greater depending on the variability of natural production (Fig. 23).

The problem thus raised is the classic but usually neglected or misunderstood problem of the (difficult) use of a Schaefer model, that could be called strategic, for tactical application. It is a strategic model because it was originally conceived by its author to help in the definition of long-term exploitation objectives (MSY, 2/3 MSY, maximum use, maximum profit, etc.). It is at present used for tactical purposes when used in the short term to calculate the level of effort or of catch to be observed the next year (fixing of quotas) .

The solution to the problem would lie in the addition to the model of another relevant variable upwelling index, for example, to make it more predictive in the short term and transform the "white noise" or unexplained variance by the classic relation into usable information. The solution could also lie in more complex simulation models.

Reduction of uncertainty in the case of unstable stocks implies, however, additional research costs and it will be important to develop procedures optimizing the risk/cost ratio.

li Abundance can be measured by acoustic surveys, for example

Catch

---_.~---..,

L - _ . . ' L -

.l....----.J...-l.---L._.B

Bmin _{B (year n)}

~~---L-...L1JL-_.B

B (year n)

clQ Potential error in determination of the quota for year n + 1

Fig. 23 Effect of interannual variability on the precision of the calculated quotas

25

The model used in California does not perhaps solve aIl the problems raised, and much progress remains to be made, in particular in countries where research does not have years of experience and data behind it.

This model has the advantage that it can be temporarily establised with incomplete data (concerning minimal biomass, for example, which could be arbitrarily fixed). The line of quotas may take various forms and allow for various strategies. It could be diffe- rent for a stock which is increasing, and for the same stock when it is regressing. The constraints and safety limits of the model and of management can be established in advance, in agreement with administration and industry, facilitating implementation in periods of sudden crisis.

5.4.3 Scientific "approximations" and management

The preceding model makes it possible ta use qualitative or approximate information concerning, for example, the form of the relation between catchability and abundance (if it is not constant, the line of determination of quotas will be a curve and not a straight line), the exact form of the model, the minimal biomass, the positive effect of the existence of a buffer biomass. The use of approximate criteria will, in these conditions, enable us to establish an approximate but flexible and relatively sound management, reducing the risks of collapse or improving the chance of reconstitution.

However, in this case as in aIl those where sorne uncertainty remalns at the level of data, the degree of approximation that the scientist may allow himself will depend on the authorities' and industry's real desire for management. If there is complete agreement (collapsed stock, for example), approximations will be easily accepted and management con- ducted at a lower research cast.

If, for various reasons, the authorities and/or industry do not really wish to take the necessary decisions, conflicts may be foreseen on the validity of the model and the parameters, and the simplicity of the approach will then become a defect. The requirement of great scientific precision in the parameters will make the model inapplicable within the given time limits, or the cast of research prohibitive.

In this sense, it may be said that the cast of usable scientific advice lS inversely proportionate ta the will to manage and to the level of risk accepted.

REFERENCES Belvèze, H.

1983

Belvèze, H.

1963

Belvèze, H.

1982 Binet, D.

1982

La pêcherie de maguereau Scomber japonicus de l'Atlantique marocain entre 29⁰N et 33 N. Paper presented to the FAO/CECAF Working Group on horse mackerel and mackerel in the northern zone of CECAF. Nouad- hibou, Mauritania, 30 January-4 February 1983. (To be published in CECAF/ECAF documents)

and K. K. Erzini. The influence of hydroclimatic factors on the availability of the sardine (S. pilchardus Walbaum) in the Moroccan Atlantic fishery.

FAO Fish.Rep./FAo-Inf.Pesca, (291)Vol.2:285-328

et al. Etat de nos connaissances sur les ressources halieutiques nationales -et-rëur niveau d'exploitation en 1982. Note Inst.Sci.pêches Marit.,

Casablanca, (3):68 p.

Influence des variations climatiques sur la pêcherie des Sardinella aurita ivoiro-ghanéennes: relation sécheresse surpêche. Océanolo.Acta, 5(4):443-52

Caverivière, A. Les espèces démersales du plateau continental ivoirien; biologie et 1982 exploitation. Thèse de doctorat d'état. Université d'Aix-Marseille II,

Faculté de Luminy, 415 p.

Crawford, R.J.M., P.A. Shelton and L. Hutchings. Aspects of variability of sorne neritic 1983 stocks in the southern Benguela system. FAO Fish.Rep./FAO Inf.Pesca,

(291)Vol.2:407-48

Csirke, J., (Chairman). Report of the Working Group on fisheries management, implications 1984 and interactions. FAü Fish.Rep.:291(1):67-90

FAO/CECAF.

1960 Fréon, P.

1983

Report of the ad hoc Working Group on Sardinella off the Coast of the Ivory Coast-Ghana-Togo. CECAF/ECAF Ser., (80/21): 73 p. Also published in French

Proùuction models as applied to substocks depending on upwelling fluctuations.

FAO Fish.Rep./FAO Inf.Pesca, (291)Vol.3:1047-66 Fox, W.W.Jr.

FRU/ORSTOM.

1976

An overview of production modelling. Collect.Vol.Sci.Rep.ICCAT/Rech.Doc.Sci.

CICTA/Colecc.Doc.Cient.CICAA,(3):142-56

Rapport du group~ de travail sur la sardinelle S. aurita des côtes ivoiro- ghanéennes. Abidjan, Côte d'Ivoire; 28 June-3 July 1976. Tema, Fish.Res.Unit et Cent.Rech.Océan., Abidjan ORSTOM, 63 p.

Garcia, S. and L. Le Reste. Life Cycles, Dynamics, Exploitation and Management of Coastal 1981 Penaeid Shrimp Stocks. FAO Fish.Tech.Pap., (203):210 p. Also published in

French

Gulland, J.A. and S. Garcia. Observed patterns in multispecies fisheries. Paper to be 1984 presented at the Dahlem Workshop on exploitation of marine communities,

Berlin, 1-6 April 1984 Hayasi, S.

1983

Sorne explanations for changes in abundance of major nerltlc pelagic stocks in the northwestern Pacific Ocean. FAO Fish.Rep./FAO Inf.Pesca, (291)Vol.2:37-56

Jones, R.

1983

27

The decline in herring and mackerel and the associated increase in other species in the North Sea. FAO Fish.Rep./FAO 1nf.Pesca, (291)Vol. 2:507-20

Jordan, R.S.

1983

Variability of pelagie stocks in the south eastern Pacifie.

1nf.Pesca, (291)Vol. 2:113-20

fAO Fish.Rep./FAO

Larkin, P.A.

1977

An epitaph for the concept of max~mum sustainable yield.

106 ( 1) : 1-1 1

Trans.Am.Fish.Soc., Laurec, A., A. Fonteneau and C. Champagr-:lt. A study of the stability of sorne stocks

1980 described by self gen~Ia(ing stochastic models. ReportP.-V.Meeting C1EM, 177 :423-38

Le Reste, L.

1984

Etuùe de variations annuelles de la production des crevettes dans l'estuaire de la Casamance (Sénégal). GFCM Stud.Rev./Etud.Rev.CGPM, (61) (To be published) MacCclll ,A.D. Population models for the northern anchovy (Engraulis mordax). Report

1980 P.-V.Meeting C1EM, 177:292-306

MacCall, A.D. Variability of pelagie fish stocks of California. FAO Fish.Rep./FAO 1nf.

1983 Pesca, (291)Vo1.2:101-12 Sharp, G.

1983

and J. Csirke, (ed.). ~roceedings of the Expert Consultation to examine changes in abundance and species composition of neritic fish resources. San Jose, Costa Rica, 18-29 April 1983. Actas de la Consulta de Expertos para examinar los cambios en la abundancia y composiciôn par especies de recursos neriticos.

San José, Costa Rica, 18-29 abril 1983. A preparatory meeting for FAO World Conference on fisheries management and development. Una reuniôn preparatôria para la Conferencia Mundial de la FAO sobre ordenaciôn y desarrollo pesqueros.

FAO Fish.Rep./FAO 1nf.Pesca, (291)Vol.2:553 p. Vol.3:557-1224

Sissenwine, M.P. 1s MSY an adequate foundation for optimum yield? Fisheries, 3(6):22-4,

1978 37-42

Soutar, A. and J.D. 1saacs. Abundancy of pelagie fish during the 19th and 20th century 1974 as recorded in anaerobic sediments of California. Fish.Bull.NOAA/NMFS,

72:257-73 Tanaka, S.

1983

Variations of pelagie fish stocks ~n waters around Japan, FAO Fish.Rep./FAO 1nf. Pesca, (291)Vol. 2: 17-36

29

Annex 1

SEE THE CASE OF UNS TABLE BIOMASSES (Annex 2)

UNDEVELOPED

- - - -

(

1 DEVELOPED

+

1 1 1

1

1

1

1

1-

1 1 1 1 1 1

t

1 1

'---<---

Interactions between management and environment ~n the case of stable stocks.

The effort is the principal factor of change.

F fishing effort q catchability 1'= increase

B biomass _C catch 'Tf= rapid increase

favourable economy? E= favourable environment?

insufficient spawning potential (overfishing of recruitment) recruitment lacking (exceptional mortality of larvae)

o

= YES

• = NO positive profitability?

effort efficiently regulated? (blocked or reduced) control of gains in efficiency of catch?

haIt to fishery investments?

=

=

=

<TI2E>

~PAW~~

=

D<1l

<§~ = sufficient spawning biomass?

(F optimized) (Fblocked) (~_..:.ontrolle~

<P

raf i ts ) a)

Annex 2

SEE CASE OF

.... y STABLE BIOMASSES (Annex 1)

DEVELOPED LOW RISK

, - - - 1

1

1

1

1 1 1 1

1

1 1 1

_J

\

\

\

\ 1 1

- - - - _ _ .J

F INCREASE

...---...,

l "'-

1

COLLAPSED

Interactio~s bet\leen nanagenent and environnent in the case of unstable Gtocks.

The environnent playa an important role. (Captions: see Annex 1)